Radio frequency directional coupler utilizing crossed coupling slots of unequal dimensions



Apr 9, 1968 R. R. KINSEY' 3,377,571

RADIO FREQUENCY DIRECTIONAL COUPLER UTILIZING CROSSED COUPLING SLOTS OF UNEQUAL DIMENSIONS Filed Dec. 27, 1965 V (ISOLATED ARM) J MQD MMD (COUPLED ARM) v INVENTORI ISOLATED RICHARD R. KINSEY,

HIS ATTORNEY,

United States Patent Office 3 ,377,571 Patented Apr. 9, 1968 RADIO FREQUENCY DIRECTIONAL COUPLER UTILIZING CROSSED COUPLING SLOTS F UNEQUAL DIMENSIONS Richard R. Kinsey, Syracuse, N.Y., assignor to General Electric Company, a corporation of New York Filed Dec. 27, 1965, Ser. No. 516,283 Claims. (Cl. 33310) This invention relates generally to radio frequency coupling devices and more particularly to cross-guide directional couplers wherein the coupling apertures are compensated dimensionally to provide improved directivity of coupling.

Cross-guide couplers as commonly constructed comprise two rectangular-section waveguides disposed at right angles with their broad walls juxtaposed to provide one common wall through which one or more apertures couple electromagnetic energy between the waveguides. These apertures normally are spaced along a diagonal to the common wall, in diagonally opposite quadrants of the common wall, and may take the form of slots, crossed slots, circular orifices or other form. In such couplers the electromagnetic wave travelling along the main waveguide is coupled through the common wall apertures into but one arm of the coupled waveguide, so that there is an electromagnetic wave induced in such coupled arm but not in the other arm, termed the isolated arm.

As is well known in the art, directivity of coupling in practical couplers can not be made perfect and the ratio of the power transmitted into the coupled arm to the power transmitted into the isolated arm, which ratio should desirably be infinite, normally does not achieve that desideratum. It is also known that the principal reason for this is that while in the isolated arm there occurs complete or nearly complete concellation of the wave induced through magnetic field coupling, this is not also true of the wave induced through electric field coupling. The uncancelled electric field coupled wave in the isolated arm reduces the directivity of coupling and thus compromises coupler performance.

The existence of this problem has been recognized for some time and a number of approaches to its alleviation have been suggested. One such proposal entails use of slots having relatively large length-to-width ratio, but this involves its own problems of coupler inefiiciency. As the slot width is reduced, the slot length must be increased to maintain the same degree of coupling. However, for tight coupling, the slots may approach their resonant length which causes a high sensitivity of coupling to frequency. Another approach involves use of additional subsidiary coupling apertures so oriented as to couple into the isolated arm a wave of proper phase and amplitude to cancel the undesired wave induced by electric field coupling. While this constitutes an effective solution to the problem it likewise is subject to space limitations and also tends to be relatively costly because of the necessity to machine or otherwise form additional apertures in the common broadwall.

The present invention is directed to cross-guide couplers of this general type and affords improved directivity of coupling while avoiding or minimizing the problems characteristic of prior approaches to directivity improvement such as those just discussed. It is, therefore, a primary object of the invention to provide cross-guide directional couplers dimensionally compensated to afiord improved directivity of coupling. It is also an object to provide improved directivity of coupling even at high coupling values, by selection of coupling slot configuration, orientation and dimensional compensation in accordance with the invention.

In carrying out the invention in one presently preferred embodiment, the two waveguides between which coupling is desired are disposed substantially perpendicular to each other and in contiguous relation so as to provide a common broadwall between them. One or more coupling apertures are provided through this common broadwall, with each such aperture comprising a pair of crossed slots oriented with their axes disposed at approximately 45 to the waveguide centerlines. In accordance with the invention the two slots are made of unequal effective lengths and so proportioned that, in the isolated arm of the coupler, the wave induced by magnetic field coupling through the slot having the longer effective length is just equal to the sum of the wave induced by magnetic field coupling through the shorter slot plus the wave induced by electric field coupling into the isolated arm. These waves then sum to zero in the isolated arm, thus yielding high directivity. For more perfect compensation in some cases the slots may be angled slightly from the 45 preferred orientation, and for maximum tightness of coupling the slots may be made of anchor configuration as hereinafter fully explained.

The invention will be further understood and its various objects, features and advantages more fully appreciated by reference to the appended claims and the following detailed description when read in conjunction with the accompanying drawings, wherein:

' FIGURE 1 is a perspective view (partially broken away) of a directional coupler incorporating the invention in a preferred embodiment;

FIGURES 2a and 2b are phasor diagrams illustrating magnetic and electric field relationships in directional couplers in accordance with the invention;

FIGURE 3 is a diagrammatic showing of a directional coupler incorporating another embodiment of the invention; and

FIGURE 4 is a diagrammatic showing of still another embodiment of the invention.

With continued reference to the drawings, wherein like reference numerals have been used throughout to designate like elements, FIGURE 1 illustrates a directional coupler of the type utilizing a single principal coupling aperture, at 11. This aperture 11 is formed in the area of broadwall 13 common to the main waveguide 15 and to the coupled guide 17, which guides are disposed at right angles and in contiguous relation as shown so as to provide such common broadwall.

In this arrangement coupling between the main and coupled waveguides occurs through the aperture 11 which is located at the point of circularly polarized magnetic field for both guides. The resultant coupling is such that an electromagnetic wave traveling along the main waveguide 15 in the direction of the arrow 19 induces in the coupled waveguide a wave which travels in the direction of the arrow 21 shown issuing from the arm of the coupler marked coupled arm. No wave would be induced in the arm marked isolated arm, were the couplerto display perfect directivity of coupling.

As previously explained, however, such perfect directivity of coupling is not actually achieved in practice, and with conyentional coupler compensation techniques it can be approached only by use of undesirably high slot lengthto-Width ratios and/or by complicating expedien'ts such as the addition of one or more subsidiary apertures so disposed as to cancel the wave induced in the isolated arm. In accordance with the invention, compensation may be more easily and economically obtained and high directivity achieved simply by orienting the coupling aperture so that the slot axes are disposed at angles of approxim-aitely 45 to the waveguide centerlines, and then making the two slots of effective length-s unequally proportioned so that there is net magnetic field coupling into the isolated arm of amplitude and phase just equal and opposite to the electric field coupling into that arm. Complete cancellation of fields induced in the isolated arm will then occur and directivity may thus be raised to the desired high level without further compensation.

The explanation and mode of accomplishment of the directivity compensation technique of this invention can best be understood by reference to FIGURES 2A and 2B, which illustrate magnetic and eleotric field phase relationships, and FIGURE 3 which more clearly shows structural relationships characteristic of couplers in accordance with the invention. Referring first to FIGURE 3, it will be noted that the coupler shown is of the two aperture type as opposed to the single aperture configuration of FIGURE 1. Directivity compensation is accomplished in substantially the same way, however, as will later become apparent.

In FIGURE 3 the directivity of coupling, defined as the ratio of coupling into the coupled arm 25 to isolation in the isolated arm 27, is optimized by arrangement of the two crossed-slot apertures 29 and 31 with their slot axes disposed at angles of approximately 45 to the waveguide centerlines, and providing the slots 29a and 31a wit-h effective lengths greater than those of the slots 29b and 31b, respectively. As shown, the slots 29b and 31b having the shorter effective lengths are disposed along one diagonal to the common broadwall area, hereinafter termed the main diagonal and designated MD in FIGURE 3, while the longer slots 29a and 2% are disposed along the quadrant diagonals QD parallel to the other common broadwall diagonal.

The result of this difierence in effective lengths of the slots of each pair can best be under-stood by reference to FIGURES 2A and 2B, which are phasor represent-ations of the magnetic and electric fields in the isolated and coupled arms, respectively. Referring first to FIG- URE 2A, the net electric field coupling into the isolated arm is represented by the phasor P; magnetic field coupling through slots 29a and 31a parallel to the quadrant diagonal QD is represented by the phasor M and magnetic field coupling through the slots 2% and 31b parallel to the main diagonal MD is represented by the phasor M As shown, the magnetic field M is opposite in phase to the phasor M and, with the slots oriented at approximately to the waveguide centerlines, also is opposite in phase to the electric field phasor P. In conventional couplers having slots of equal effective lengths the two magnetic field phasors M and M are of equal amplitude and being of opposite phase are effective to cancel each other. The electric field phasor P is not cancelled, however, so wave energy is transmitted into the isolated arm by uncancelled electric field coupling, thus reducing directivity. In the couplers of this invention, the unequal effective lengths of the slots of each pair results in a difference in magnitude of the M and M phasors, and by proper choice of the differential in slot length the difference in magnetic field phasor magnitude may be made precisely equal andopposite to the electric field phasor P, so that there is complete cancellation and no net magnetic or electric field coupling into the isolated arm.

In the coupled arm there results a similar inequality'of the M and M phasors, but in this arm the magnetic field phasors are coincident in phase with each other and 90 out of phase with the electric field phasor. There accordingly is no similar cancellation of fields in this arm, and coupling into it is not significantly affected. In those couplers wherein desired coupling is limited by spaceimposed limitations on slot length, the necessity to make one slot of each pair shorter than the other may result in some slight reduction in coupling, of course, but generally this is small and it may be made negligible by use of the anchor slot configuration described hereinafter.

The difference in slot length necessary to achieve compensation is itself not large, normally falling within the range of from 7 to 20 percent. The precise value within this range best for optimized compensation is easily determined by simple cut-and-try techniques, and is dependent on such factors as length/width ratios of the slots. In this connection it should be noted that dimensional compensation also is possible by making the slots of equal length but of unequal width. This adjustment has a first order effect on both electric and magnetic fieldcoupling, however, and accordingly is not as desirable as is compensation by difference of slot length alone.

The phase relationships depicted in FIGURES 2A and 2B are known to be precisely true only in the limit for small coupling apertures, and for finite slots departures from these ideal relationships are to be expected. However, since the phasors of the magnetically coupled wave can be rotated with respect to that of the electrically coupled wave by rotating the slots from their 45 orientation with respect to the waveguide centerlines, substantially complete cancellation stillcan be obtained by slight rotation of the slots about their centers to a position only approximating the 45 orientation shown. Experimentally it has not been found necessary to depart even to this extent from the 45 orientation in order to obtain fully satisfactory compensation.

Referring now to FIGURE 4, there is illustrated a cross-guide coupler wherein the coupling apertures 35 and 37 through the common broadwall39 are of anchor configuration. The design criteria for this slot configuration and its several important advantages in directional couplers are fully disclosed in copending application S.N. 334,169, filed Dec. 30, 1963, now patent No. 3,230,483 issued J an. 18, 1966 to the present inventor.

The anchor slot configuration offers particular advantage in couplers in which directivity improvement by dimensional compensation is to be effected, because it ena'bles such compensation in any of several ways and with little adverse affect on coupling. In the embodiment of FIGURE 4, for example, the necessary inequality in effective lengths of the slots of each pair can be accomplished by making the lengths of the centermost fluke arm portions of one slot of each pair relatively shorter than the fluke arm portions of the other slot of that pair.

This reduces the effective length of the short fluke slot as explained in the copending application identified above. By such variation of relative lengths of the slot fluke portions it is possible to achieve essentially the same compensation as would resultzfrom a change in length of the main body portion of the slot, without encountering space limitations and other problems, which achange in slot overall length might entail. It may be desirable in particular applications to use adjustment of slot overall length in lieu of or in addition to adjustment of fluke arm length, and also to adjust fluke arm length on both ends of the slots disposed along the main diagonal rather than on the centerm-ost ends only as shown.

Certain preferred embodiments of the invention have been described and illustrated by way of example in the foregoing, but many modifications will occur to those skilled in the art and it therefore should be understood that the appended claims are intended to cover all such modifications as fall within the true spirit and scope of the invention.

What I claim is: 1. Means for directionally coupling radio frequency wave energy between waveguides comprising: i

a main waveguide and a coupled waveguide with the latter including a coupled arm extending in the direction of desired wave coupling and an isolated arm extending in the opposed direction, said waveguides being disposed in contiguous relation so as to define a common wall area between. said isolated and coupled arms of said coupled waveguide; said common wall having a coupling aperture therethrough comprising a pair of crossed slots, said slots being of unequal effective dimensions so proportioned as to provide substantially complete cancellation of wave energy otherwise transmitted by magnetic and electric field coupling through said slots into said isolated arm.

2. Means for coupling radio frequency energy in desired direction comprising:

main and coupled waveguides having surfaces disposed in contiguous relation so as to define a common wall and having their centerlines disposed so as to divide said common wall into substantially rectangular quadrants;

said common wall having formed in at least one quadrant thereof a coupling aperture including first and second slots arranged in crossed relation such that said first slot has its longitudinal axis disposed along one diagonal of said common wall and said second slot has its longitudinal axis disposed along the quadrant diagonal parallel to the other common wall diagonal;

said second slot being dimensionally larger than said first slot so as to establish a differential of magnetic field coupling through the two slots equal and opposite to electric field coupling therethrough in the direction opposite to that of desired coupling.

3. Means for directionally coupling radio frequency waves between waveguides comprising:

a main waveguide and a coupled waveguide including an isolated arm and a coupled arm, said guides being disposed in contiguous relation so as to together define a common wall area between said isolated and coupled arms and so that their centerlines divide said common wall area into substantially rectangular quadrants;

said common wall having formed in at least one quadrant thereof a coupling aperture including first and second slots arranged in crossed relation such that said first slothas its longitudinal axis disposed along one diagonal of said common wall and said second slot has its longitudinal axis disposed along the quadrant diagonal parallel to the other common wall diagonal;

said first slot being of effective length sufficiently shorter than said second slot that the wave induced in said isolated arm by magnetic field coupling through said first slot summed with the wave induced by electric field coupling into said isolated arm is substantially equal in amplitude and opposite in phase to the wave induced in said isolated arm by magnetic field coupling through said second slot, thus effecting wave cancellation in said isolated arm.

4. Directional coupling means as defined in claim 3 wherein said slots are of anchor configuration and the anchor fluke arms on at least one end of said first slot are shorter than those on said second slot to thus afford the latter its relatively greater effective length.

5. Directional coupling means as defined in claim 3 wherein at least one of said slots is rotated from parallelism with the diagonal along which it is disposed, in order that the waves coupled through said slots into said isolated arm be more closely opposite in phase and more completely cancel.

No References Cited.

HERMAN KARL SAALBACH, Primary Examiner. M. NUSSBAUM, Assistant Examiner. 

1. MEANS FOR DIRECTIONALLY COUPLING RADIO FREQUENCY WAVE ENERGY BETWEEN WAVEGUIDES COMPRISING: A MAIN WAVEGUIDE AND A COUPLED WAVEGUIDE WITH THE LATTER INCLUDING A COUPLED ARM EXTENDING IN THE DIRECTION OF DESIRED WAVE COUPLING AND AN ISOLATED ARM EXTENDING IN THE OPPOSED DIRECTION, SAID WAVEGUIDES BEING DISPOSED IN CONTIGUOUS RELATION SO AS TO DEFINE A COMMON WALL AREA BETWEEN SAID ISOLATED AND COUPLED ARMS OF SAID COUPLED WAVEGUIDE; SAID COMMON WALL HAVING A COUPLING APERTURE THERETHROUGH COMPRISING A PAIR OF CROSSED SLOTS, SAID SLOTS BEING OF UNEQUAL EFFECTIVE DIMENSIONS SO PROPORTIONED AS TO PROVIDE SUBSTANTIALLY COMPLETE CANCELLATION OF WAVE ENERGY OTHERWISE TRANSMITTED BY MAGNETIC AND ELECTRIC FIELD COUPLING THROUGH SAID SLOTS INTO SAID ISOLATED ARM. 